A little history, Giuliano:
TomTom has tended to provide as much map as would fit within a given memory model. Back quite some years, for example, there were some units with so little memory that a UK/ROI map was all that would reasonably fit. At the same time, there were higher end units with (wow!) a whole 2GB of memory available, and even one with 4GB available so as to hold both Western Europe and North America at the same time. Now, a 4GB unit barley holds either of those. Not all units had an SD or uSD card slot that might have helped, and this is back when 2GB was the maximum card size anyway (SDHC had not yet been made available). In any case, a map could never be split between internal and external (card) memory, so the maximum remained 2GB for that time.
'Europe' as a whole was always to big for any of the units they were selling, and that is why they provided some overlapping European maps ... Western, Central and Eastern. They also supplied some regional maps (e.g., the UK/ROI or Iberia or similar) for the smallest models, or as add-on maps for those outside of those areas. But the cost to buy these ala carte was expensive.
Meanwhile, maps like Western Europe continued to grow with more roads and more detail, and then those didn't fit, not even in 2GB units. Understand that different models got somewhat different maps with more or less detail, and TomTom kept trying to find ways to make maps fit into the older models. It was at that time that 'zones' were created to segment the older map sets into yet smaller pieces. TomTom figured you had paid for access to, for example, all the countries in a Western Europe map, so they decided that they owed you a way to access them, even if all of Western Europe wouldn't fit into your unit at the same time. Later, this happened in North America as well. Where they used to be able to fit Canada/USA/Mexico/Guam into a single 2GB map, that is no longer the case for many units, and they started to segment North America into 'zones' as well.
The newer Nav4 units (even the less expensive ones) are now 8GB, and will again hold a full Western Europe or North America map -- for now. Who knows? There may be a day when they also grow so large as to require an external 16GB uSD card. That's another difference: all Nav4 devices have an external uSD card slot.
There are several reasons why TomTom has not always provided many small maps. [Problem: I have some things I need to do right away, and I can't finish this now. But here's the start of it:]
First, it is necessary to understand that a while back, neither TomTom nor Garmin owned a map making business. TomTom would buy their maps from Teleatlas, and Garmin would buy their maps from Navteq. Both Teleatlas and Navteq had other customers to service as well. It wasn't until some years into the program (2008) that TomTom bought Teleatlas. Garmin did not choose to try to buy Navteq, and in fact, in 2007, Nokia began discussions to buy Navteq (possibly why TomTom was prompted to purchase Teleatlas?) and that deal was also completed in 2008.